This invention relates to an optical lens and a method of producing the same, a method of producing an optical lens array, a focus error signal production method, and an optical pickup apparatus, and more particularly to an optical lens for use with an optical system of an optical disk apparatus or the like, a method of producing the same and a method of producing an optical lens array for use with an optical system of an optical disk apparatus or the like, a focus error signal production method for an optical disk apparatus, and an optical pickup apparatus suitable for use with an optical disk apparatus.
In recent years, in the field of information recording, much effort has been and is being directed to investigations for an optical information recording system. The optical information recording system is advantageous at least in that it allows recording and playback without contact, that it can achieve a recording density higher by one or more digit than that by a magnetic recording system, and that it can be applied to various memory types including the reproduction-only type, the write-once type and the erasable type. Therefore, the optical information recording system is expected as a system which allows realization of a less expensive file of a large capacity and can be used for various applications from industrial applications to consumer applications.
In an optical pickup apparatus incorporated in a recording and/or playback apparatus (hereinafter referred to sometimes as optical disk apparatus) for an optical recording medium (hereinafter referred to sometimes as optical disk) such as a compact disk (CD) or a digital versatile disk (DVD) for such various optical information recording systems as described above, laser light from a laser diode which emits a laser beam of a wavelength of, for example, 780 nm or 650 nm is condensed on an optical recording layer of an optical disk by an optical system including an optical member such as a beam splitter. Reflected light from the optical disk follows a reverse route through the optical system and is projected on a light receiving element such as a photodiode by a multi-lens device.
Information recorded on the optical recording layer of the optical disk is thus read out depending upon the variation of the reflected light from the optical disk.
In the optical pickup apparatus described above, since the distance between an objective lens for condensing the laser light upon the optical recording layer of the optical disk and the optical disk has an influence on the diameter of a spot of the laser light on the optical recording layer, the relative position of the objective lens to the optical disk is adjusted by a focusing servo mechanism.
The focusing servo mechanism detects a focus error signal from the reflected light from the optical disk and moves an electromagnetic actuator, to which the objective lens is secured, based on the resulting focus error signal.
As a method of detecting the focus error signal, an astigmatism method is used widely. According to the astigmatism method, when the reflected light from the optical disk is condensed by an optical lens in order to project the light on a photodiode as a light receiving element, the focal length on a first plane of the optical lens including an axis in a focusing direction and the focal length on a second plane of the optical lens perpendicular to the first plane and intersecting with the first plane along the axis in the focusing direction are made different from each other.
With the astigmatism method, the polarity of the focus error signal differs whether the relative position of the objective lens to the optical disk is relatively far or near, and signals having the opposite polarities can be obtained.
Such an optical lens having different focal lengths on first and second planes perpendicular to each other as described above can be obtained, for example, by combining a cylindrical lens and a circular convex lens or by inserting, into a converging optical system for which a circular convex lens for converging light is used, a flat plate in an inclined relationship to an optical axis of the optical system.
However, the conventional method of realizing an optical system whose focal length is different on the first and second planes perpendicular to each other is disadvantageous in that, since it uses a plurality of optical members, a great number of parts are involved and also a great number of locations where adjustment of a part is required are involved, which makes a factor of raising the production cost. The conventional method is disadvantageous also in that the use of a plurality of optical members makes the light path of the converging optical system long, which makes miniaturization of the optical pickup apparatus difficult.
It is an object of the present invention to provide an optical lens by which an optical system whose focal length is different on first and second planes perpendicular to each other can be realized by a single optical member and hence with a low cost and which can be used to form a converging optical system of a light path of a reduced length.
It is another object of the present invention to provide a method of producing an optical lens by which an optical system whose focal length is different on first and second planes perpendicular to each other can be realized by a single optical member and hence with a low cost can be produced readily at a low cost.
It is a further aspect of the present invention to provide an optical lens array wherein a plurality of optical lenses described above are arrayed, a focus error signal production method using above-described optical lens, and an optical pickup apparatus which incorporates the optical lens.
In order to attain the objects described above, according to an aspect of the present invention, there is provided an optical lens, including a substrate made of an optical material which has a continuous flat face on a first side through which an axis in a focus direction of the the optical lens passes, and a convex element formed integrally with the substrate and having a convex curved face that protrudes from a second side of the substrate opposite from the first side so as to have a function as an optical lens, the curved face of the convex element having a first curvature on a first cross section including an axis in a focus direction of the optical lens and a second curvature on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction, the second curvature being different from the first curvature.
Preferably, a focal length of the optical lens on the first cross section and a focal length of the optical lens on the second cross section are different from each other.
Preferably, the shapes of the convex element on the first and second cross sections are symmetrical with respect to the axis in the focus direction.
Preferably, the shapes of the convex element on the first and second cross sections are shapes of arcs substantially of ellipses.
Preferably, the substrate has a flat face on the second side thereof surrounding the convex element.
Preferably, a groove is formed in the second side of the substrate along a boundary between the flat face on the second side of the substrate and the convex element. The groove may have a substantially elliptical shape or a substantially rectangular shape.
In the optical lens, the convex element which is formed integrally with the substrate and has a convex curved face so as to have a function as an optical lens is shaped such that the curvature on the first cross section including the axis in the focus direction of the optical lens and the curvature on the second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other. Consequently, the focal length on the first cross section and the focal length on the second cross section are different from each other.
Accordingly, an optical system having different focal lengths on first and second planes perpendicular to each other can be realized by a single optical member. Consequently, the production cost of the optical system can be suppressed and the length of the light path of a converging optical system of the optical system can be reduced.
According to another aspect of the present invention, there is provided a method of producing an optical lens, including the steps of forming, on a substrate made of an optical material, a mask layer corresponding to a shape of an optical lens having a pattern whose width in a first direction is different from a width thereof in a second direction perpendicular to the first direction, deforming the mask layer by heat treatment so that a surface area of the mask layer may be reduced, and removing the mask layer and the substrate simultaneously to transfer the shape of the mask layer to the substrate to form the shape of the optical lens.
Preferably, the mask layer is formed by performing a light-exposure and development process for a photosensitive material film to pattern the photosensitive material film. More preferably, in the step of deforming the mask layer by heat treatment so that a surface area of the mask layer may be reduced, the heat treatment is performed at a temperature higher than a glass transition point of the photosensitive material film. Also in the step of deforming the mask layer by heat treatment so that a surface area of the mask layer may be reduced, the heat treatment is performed at a temperature lower than a carbonization temperature of the photosensitive material film.
Preferably, in the step of deforming the mask layer by heat treatment so that a surface area of the mask layer may be reduced, the heat treatment is performed at a temperature higher than a room temperature.
Preferably, in the step of removing the mask layer and the substrate simultaneously, a dry etching process is performed using the mask layer as a mask to transfer the shape of the mask layer to the substrate to form the shape of the optical lens. More preferably, the dry etching process is performed in a condition that selection ratios for the substrate and the mask layer are substantially equal to each other.
According to the method of producing an optical lens, a light-exposure and development process is performed for a photosensitive material film to pattern the photosensitive material film to form, on a substrate made of an optical material, a mask layer corresponding to a shape of an optical lens having a pattern whose width in a first direction is different from a width thereof in a second direction perpendicular to the first direction.
Then, heat treatment is performed at a temperature lower than a carbonization temperature of the photosensitive material film but higher than a glass transition point of the photosensitive material film and the room temperature so that a surface area of the mask layer may be reduced.
Thereafter, a dry etching process or the like is performed to remove the mask layer and the substrate simultaneously to transfer the shape of the mask layer to the substrate to form the shape of the optical lens.
With the method of producing an optical lens, an optical lens wherein a convex element which is formed integrally with a substrate and has a convex curved face so as to have a function as an optical lens is shaped such that the curvature on a first cross section including an axis in a focus direction of the optical lens and the curvature on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other whereby the focal length on the first cross section and the focal length on the second cross section are different from each other can be produced.
With optical lens produced in such a manner as described above, an optical system having different focal lengths on first and second planes perpendicular to each other can be realized by a single optical member. Consequently, the production cost of the optical system can be suppressed and the length of the light path of a converging optical system of the optical system can be reduced.
According to a further aspect of the present invention, there is provided a method of producing an optical lens array, including the steps of forming, on a substrate made of an optical material, a plurality of mask layer portions corresponding to shapes of a plurality of optical lenses each having a pattern whose width in a first direction is different from a width thereof in a second direction perpendicular to the first direction, deforming the mask layer portions by heat treatment so that a surface area of each of the mask layer portions may be reduced, and removing the mask layer portions and the substrate simultaneously to transfer the shapes of the mask layer portions to the substrate to form the shapes of the optical lenses.
According to the method of producing an optical lens array, on a substrate made of an optical material, a plurality of mask layer portions corresponding to shapes of a plurality of optical lenses each having a pattern whose width in a first direction is different from a width thereof in a second direction perpendicular to the first direction are formed.
Then, the mask layer portions are deformed so that a surface area of each of the mask layer portions may be reduced.
Thereafter, the mask layer portions and the substrate are removed simultaneously to transfer the shapes of the mask layer portions to the substrate to form the shapes of the optical lenses.
With the method of producing an optical lens array, an optical lens array wherein a plurality of optical lenses are arrayed and each formed such that a convex element which is formed integrally with a substrate and has a convex curved face so as to have a function as an optical lens is shaped such that the curvature on a first cross section including an axis in a focus direction of the optical lens and the curvature on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other whereby the focal length on the first cross section and the focal length on the second cross section are different from each other can be produced.
The optical lens array produced in such a manner as described above may be divided into individual optical lenses according to the present invention.
According to a still further aspect of the present invention, there is provided a focus error signal production method for irradiating light upon an optical recording medium to obtain focus information of returning light from the optical recording medium, including the step of obtaining focus information of the returning light from the optical recording medium using an optical lens which includes a substrate made of an optical material and a convex element formed integrally with the substrate and having a convex curved face so as to have a function as an optical lens, the convex element being configured such that a focal length on a first cross section including an axis in a focus direction of the optical lens and a focal length on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other.
Preferably, the shapes of the convex element on the first and second cross sections of the optical lens are symmetrical with respect to the axis in the focus direction.
Preferably, the shapes of the convex element on the first and second cross sections of the optical lens are shapes of arcs substantially of ellipses.
Preferably, the substrate of the optical lens has a flat face on which the convex element is formed.
Preferably, a groove is formed along a boundary between the substrate and the convex element of the optical lens. Further preferably, the groove of the optical lens has a substantially elliptical shape or a substantially rectangular shape.
According to the focus error signal production method, an optical lens wherein a convex element which is formed integrally with a substrate and has a convex curved face so as to have a function as an optical lens is shaped such that the curvature on a first cross section including an axis in a focus direction of the optical lens and the curvature on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other whereby the focal length on the first cross section and the focal length on the second cross section are different from each other can be used to converge returning light from the optical recording medium to obtain focus information.
With the focus error signal production method, an optical system for which a plurality of optical members are conventionally required to produce a focus error signal by an astigmatism method can be realized with a single optical member.
According to a yet further aspect of the present invention, there is provided an optical pickup apparatus which irradiates light upon an optical recording medium and receives reflected light from the optical recording medium, including a light emitting element for emitting light, a light receiving element for receiving the light emitted from the light emitting element, and an optical member for introducing the light emitted from the light emitting element so as to be irradiated upon the optical recording medium and introducing the reflected light from the optical recording medium to the light receiving element, the optical member including an optical lens which includes a substrate made of an optical material and a convex element formed integrally with the substrate and having a convex curved face so as to have a function as an optical lens, the convex element being configured such that a focal length on a first cross section including an axis in a focus direction of the optical lens and a focal length on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other.
Preferably, the shapes of the convex element on the first and second cross sections of the optical lens are symmetrical with respect to the axis in the focus direction.
Preferably, the shapes of the convex element on the first and second cross sections of the optical lens are shapes of arcs substantially of ellipses.
Preferably, the substrate of the optical lens has a flat face on which the convex element is formed.
Preferably, a groove is formed along a boundary between the substrate and the convex element of the optical lens. More preferably, the groove of the optical lens has a substantially elliptical shape or a substantially rectangular shape.
With the optical pickup apparatus, an optical lens wherein a convex element which is formed integrally with a substrate and has a convex curved face so as to have a function as an optical lens is shaped such that the curvature on a first cross section including an axis in a focus direction of the optical lens and the curvature on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other whereby the focal length on the first cross section and the focal length on the second cross section are different from each other is used. Therefore, an optical system for which a plurality of optical members are conventionally required to produce a focus error signal by an astigmatism method can be realized with a single optical member. Consequently, the production cost can be suppressed and the light path of the converging optical system can be reduced.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.